Project 4 - Project summary: Geoscience research conducted under Project 4 focuses on the vulnerability of aquifers that are currently low in arsenic (As) to human perturbations: irrigation pumping and municipal pumping. These low As aquifers are crucial for reducing As exposure for the Health of Effects of Arsenic Longitudinal Study (HEALS) cohort in Bangladesh, and throughout South and Southeast Asia more generally. The approach builds on the premise that: a) iron oxide dissolution and precipitation control the partitioning of As between groundwater and aquifer sediments; and b) the supply of reactive organic carbon controls iron oxide dissolution and therefore the release of As to groundwater. Hypotheses to be tested in collaboration with other Projects and Cores of the Columbia University Superfund Research Program include H1: Irrigation pumping for growing rice has redistributed As in shallow aquifers of the HEALS area by enhanced lateral mixing or dilution with recharge combined with enhanced release from aquifer sands; H2: Clay layers releasing organic carbon to deep, depressurized sandy aquifers pose a greater threat of contamination in the HEALS cohort area than downward advection due to the lack of such clays, and H3: Exposure to As in the HEALS area can be predicted at the individual level from information available to the household including the status with respect to As of its own and surrounding well(s), access to these wells, and awareness to the health risks of continued exposure.